Method for producing scratch resistant, weatherproof coatings

ABSTRACT

A process for producing scratch-resistant coatings comprises applying at least one UV-curable coating composition comprising as its photochemically crosslinkable constituent 
     at least one aliphatic urethane (meth)acrylate prepolymer PU having at least two double bonds per molecule, or 
     a mixture of at least one urethane (meth)acrylate prepolymer PU and at least one reactive diluent, 
     to the target substrate and curing the resulting wet coating by exposure to ultraviolet radiation under an inert gas atmosphere.

The present invention relates to a process for producingscratch-resistant weathering-stable coatings on the basis of UV-curablecoating compositions.

Coating compositions which cure by UV radiation are used in industry toproduce high-quality coatings. Radiation-curable coating compositionsare generally flowable formulations based on polymers or oligomerscontaining crosslinking-active groups which on exposure to UV radiationundergo a crosslinking reaction with one another. This results in theformation of a high molecular mass network and thus in the developmentof a solid polymeric film. Unlike the heat-curable coating compositionsoften used to date, radiation-curable coating compositions may be usedfree from solvents or dispersants. They are further notable for veryshort curing times, which is particularly advantageous in the case ofcontinuous processing on coating lines.

Coating compositions curable by UV radiation generally give high surfacehardness and good chemical resistance. For some time there has been adesire for coatings which possess high scratch resistance, so that whenit is cleaned, for example, the coating is not damaged and does not loseits gloss. At the same time, the coatings should retain the propertiesnormally achieved with radiation-cured coatings.

In the literature there have been various descriptions of the physicalprocesses involved in the appearance of scratches and the relationshipsbetween scratch resistance and other physical parameters of the coating(on scratch-resistant coatings cf., e.g., J. L. Courter, 23^(rd) AnnualInternational Waterborne, High-Solids and Powder Coatings Symposium, NewOrleans 1996).

A variety of test methods have been described to quantify the scratchresistance of a coating. Examples include testing by means of the BASFbrush test (P. Betz and A. Bartelt, Progress in Organic Coatings 22(1993) 27-37), by means of the AMTEC wash brush installation, or varioustest methods analogous to scratch hardness measurements, as describedfor example by G. Jüttner, F. Meyer, G. Menning, Kunststoffe 88 (1988)2038-42. A further test to determine scratch resistance is described inEuropean Coatings Journal 4/99, 100 to 106.

In accordance with the present state of development, three routes toscratch-resistant surfaces are being discussed, which in principle mayalso be transferred to UV-curing systems.

The first route is based on increasing the hardness of the coatingmaterial. Since a harder material cannot be scratched by a softer one, ahigh level of hardness is a sufficient prerequisite for scratchresistance. However, the high level of hardness is at the expense ofother properties, such as the penetration depth or the adhesion, whichare vital to coating materials.

The second route is based on selecting the coating material such that onscratching it is stressed in the reversible deformation range. Thematerials involved are those which permit high reversible deformation.However, there are limits on the use of elastomers as coating materials.Coatings of this kind usually exhibit poor chemical stability. Thisapproach has to date played no part in practical application.

A third approach attempts to produce coatings having a ductile, i.e.,plastic deformation behavior and at the same time to minimize the shearstress within the coating material that occurs on scratching. This isdone by reducing the friction coefficient, using waxes or slipadditives, for example. Coatings additives for UV-curing systems aredescribed, for example, in B. Hackl, J. Dauth, M. Dreyer; Farbe & Lack103 (1997) 32-36.

U.S. Pat. No. 5,700,576 describes a UV-curing, scratch-resistant coatingwhich comprises 1-30% by weight of a prepolymeric thickener containingthiol groups and 20-80% by weight of one or more polyfunctionalacrylates or methacrylates, and also diluents, especially reactivediluents containing a free-radically polymerizable group, free-radicalinitiators, and further customary additives for producing coatings. Thepolymerization and thus curing of the coating is initiated byirradiation with UV light.

EP 0 544 465 B1 describes a UV-curable, scratch-resistant coating whosehardness is increased by incorporating colloidal silica into the coatingmatrix, the intention being at the same time to retain the flexibilityof the organic matrix. A scratch-resistant coating of this kind contains1-60% by weight of colloidal silica, 1-50% by weight of the hydrolysisproduct of an alkoxysilyl acrylate, especially3-methacryloyloxypropyltrimethoxysilane, and 25-90% by weight ofacrylate monomers, comprising a mixture of 20-90% by weight ofhexanediol diacrylate and 10-80% by weight of a monofunctional branchedor carbocyclic acrylate, and also a sufficient amount of free-radicalinitiator.

It is an object of the present invention to provide a process forproducing scratch-resistant and weathering-stable coatings on the basisof UW-curable coating compositions.

We have found that this object is achieved, surprisingly, by a processwhich involves applying a liquid, UV-curable coating composition basedon aliphatic urethane (meth)acrylate prepolymers to the target substrateand curing the still-wet coating subsequently by UV radiation in thesubstantial absence of oxygen.

The present invention accordingly provides a process for producingscratch-resistant coatings, which involves applying at least oneUV-curable coating composition comprising as its photochemicallycrosslinkable constituent

at least one aliphatic urethane (meth)acrylate prepolymer PU having atleast two double bonds per molecule, or

a mixture of at least one urethane (meth)acrylate prepolymer PU and atleast one reactive diluent,

to the target substrate and curing the resulting wet coating by exposureto ultraviolet radiation under an inert gas atmosphere.

The term inert gas atmosphere refers to an essentially oxygen-freeatmosphere of chemically inert gases, such as nitrogen, carbon-monoxide,carbon dioxide and noble gases, e.g., argon, or mixtures of said gases.Inert gases generally contain not more than 2000 ppm of impurities andnormally not more than 500 ppm of oxygen. These slight traces of oxygendo not impair the effect of the invention. Even amounts of oxygen of upto 2% by volume do not detract from the effect of the invention. Highergrades of nitrogen contain less than 10 ppm of oxygen. Typical argongrades contain less than 6 ppm of oxygen. The preferred inert gas isnitrogen.

The UV-curable coating compositions used in the process of the inventioncomprise as their photochemically crosslinkable constituents and,accordingly, their film formers at least one aliphatic urethane(meth)acrylate prepolymer PU having at least two double bonds permolecule, or a mixture of such prepolymers PU with at least one reactivediluent, selected preferably from difunctional and polyfunctional estersof acrylic acid and/or of methacrylic acid with aliphatic diols orpolyols (reactive diluent R).

Aliphatic urethane (meth)acrylate prepolymers are polymeric oroligomeric compounds which have urethane groups and acryloxyalkyl and/ormethacryloxyalkyl groups or (meth)acrylamidoalkyl groups. Normally, the(meth)acryloxyalkyl and/or (meth)acrylamidoalkyl groups are attached viathe oxygen atom of the urethane group. The term acryloxyalkyl groupsrefers to C₁-C₁₀ alkyl radicals, preferably C₂-C₅ alkyl radicals,substituted by one, two or three, preferably one, acryloxy group.Similar comments apply to methacryloxyalkyl groups. Accordingly,(meth)acrylamidoalkyl groups are C₁-C₁₀ alkyl radicals, preferably C₂-C₅alkyl radicals, substituted by one, two or three (meth)acrylamidogroups, preferably by one (meth)acrylamido group. In accordance with theinvention, the aliphatic urethane (meth)acrylate prepolymers PU have atleast two double bonds per molecule, preferably from three to six doublebonds per molecule. The aliphatic urethane (meth)acrylate prepolymers PUof the invention are essentially free from aromatic structural elements,such as phenylene or naphthylene or substituted phenylene or naphthylenegroups.

The urethane (meth)acrylate prepolymers or oligomers PU used as bindersnormally have a number-average molecular weight M_(N) in the range from500 to 5000, preferably in the range from 500 to 2000 daltons (asdetermined by means of GPC on the basis of authentic comparisonsamples). The double bond equivalent weight (g of polymer per doublebond present therein) is preferably in the range from 250 to 2000 and inparticular in the range from 300 to 900.

The prepolymers PU or mixtures thereof with the reactive diluent R thatare employed in accordance with the invention preferably have aviscosity (as determined using a rotational viscometer in accordancewith DIN EN ISO 3319) in the range from 250 to 11,000 mPa.s, inparticular in the range from 2000 to 7000 mPa.s.

The aliphatic urethane (meth)acrylate prepolymers are known in principleto the skilled worker and may be prepared, for example, as described inEP-A-203 161. The content of that document, insofar as it relates tourethane (meth)acrylate prepolymers and their preparation, is herebyincorporated fully by reference.

Urethane (meth)acrylate prepolymers preferred for use in accordance withthe invention are obtainable by reacting at least 25% of the isocyanategroups of a compound (component A) containing isocyanate groups with atleast one hydroxyalkyl ester of acrylic acid and/or of methacrylic acid(component B) and, if desired, with one or more further compounds(component C) having at least one functional group which is reactivetoward isocyanate groups.

The relative amounts of components A, B and C are preferably chosen suchthat

1. the ratio of equivalents of the isocyanate groups in component A tothe reactive groups in component C is between 3:1 and 1:2, preferablybetween 3:1 and 1.1:1, and in particular about 2:1, and

2. the hydroxyl groups of component B correspond to the stoichiometricamount of the free isocyanate groups of component A, i.e., to thedifference between the total number of isocyanate groups of component Aminus the reactive groups of component C (or minus its reacted reactivegroups of component C if only partial reaction of the reactive groups isintended).

Preferably, the binder contains no free isocyanate groups. In oneadvantageous embodiment, therefore, component B is reacted in astoichiometric ratio with the free isocyanate groups of component A.

The urethane (meth)acrylate prepolymers may also be prepared by firstreacting some of the isocyanate groups of a low molecular massdiisocyanate or polyisocyanate, as component A, with at least onehydroxyalkyl ester of an ethylenically unsaturated carboxylic acid, ascomponent B, and subsequently reacting the remaining isocyanate groupswith a chain extender (component C1). In this case it is also possibleto use mixtures of chain extenders. In this case also, the relativeamounts of components A, B and C are chosen such that the ratio ofequivalents of the isocyanate groups to the reactive groups of the chainextender is between 3:1 and 1:2, preferably 2:1, and the ratio ofequivalents of the remaining isocyanate groups to the hydroxyl groups ofthe hydroxyalkyl ester is 1:1.

Compounds containing isocyanate groups are understood, here and below,to be low molecular mass, aliphatic diisocyanates or polyisocyanates andalso aliphatic polymers or oligomers containing isocyanate groups(prepolymers) having at least two and preferably from three to six freeisocyanate groups per molecule. The boundary between the low molecularmass diisocyanates or polyisocyanates and the prepolymers containingisocyanate groups is fluid. Typical prepolymers containing isocyanategroups generally have a number-average molecular weight M_(n) in therange from 300 to 5000 daltons, preferably in the range from 400 to 2000daltons. The low molecular mass diisocyanates or polyisocyanatespreferably have a molecular weight of less than 400 daltons, inparticular of less than 300 daltons.

Typical aliphatic diisocyanates or polyisocyanates of low molecular massare tetramethylene diisocyanate, hexamethylene diisocyanate,octamethylene diisocyanate, decamethylene diisocyanate, dodecamethylenediisocyanate, tetradecamethylene diisocyanate,1,6-diisocyanato-2,2,4-trimethylhexane,1,6-diisocyanato-2,2,4,4-tetramethylhexane, 1,2-, 1,3- or1,4-diisocyanatocyclohexane, 4,4′-di(isocyanatocyclohexyl)methane,1-isocyanato-3,3,5-trimethyl-5-(isocyanatomethyl)cyclohexane(isophoronediisocyanate), 2,4- or 2,6-diisocyanato-1-methylcyclohexane, and alsothe biurets, cyanurates and allophanates of the abovementioneddiisocyanates.

The polyisocyanates containing isocyanurate groups comprise, inparticular, simple triisocyanato isocyanurates, which represent cyclictrimers of the diisocyanates, or comprise mixtures with their higherhomologs having more than one isocyanurate ring. Mention may be madehere by way of example of the isocyanurate of hexamethylene diisocyanateand of the cyanurate of toluene diisocyanate, which are availablecommercially. Cyanurates are used preferably in preparing urethane(meth)acrylates.

Oligomers and polymers containing isocyanate groups are obtainable, forexample, by reacting one of the abovementioned low molecular massdiisocyanates or polyisocyanates with a compound having at least twofunctional groups that are reactive toward isocyanate groups. Compoundsof this kind are also referred to as chain extenders (component C1) andare included in component C.

Suitable hydroxyalkyl esters of acrylic acid and of methacrylic acid(component B) are the monoesters of acrylic acid and, respectively, ofmethacrylic acid with C₂-C₁₀ alkanediols, such as 2-hydroxyethylacrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl acrylate,3-hydroxypropyl methacrylate, 4-hydroxybutyl acrylate, and4-hydroxybutyl methacrylate. As well as or in addition to thehydroxyalkyl esters of acrylic acid and/or of methacrylic acid it isalso possible to use other hydroxyl-containing esters of acrylic acidand/or of methacrylic acid in order to introduce double bonds into theurethane (meth)acrylate prepolymer PU, such as trimethylolpropanediacrylate or dimethacrylate, and also hydroxyl-carrying amides ofacrylic acid and of methacrylic acid, such as 2-hydroxyethylacrylamideand 2-hydroxyethylmethacrylamide.

Suitable chain extenders (component C1) are aliphatic diols or polyolshaving up to 20 carbon atoms, such as ethylene glycol, diethyleneglycol, propylene glycol, dipropylene glycol, 1,4-butanediol,1,5-pentanediol, neopentyl glycol, 1,6-hexanediol,2-methyl-1,5-pentanediol, 2-ethyl-1,4-butanediol,2,2-bis(4′-hydroxycyclohexyl)propane, dimethylolcyclohexane, glycerol,trimethylolethane, trimethylolpropane, trimethylolbutane,pentaerythritol, trimethylolpropane, erythritol and sorbitol; diaminesor polyamines having up to 20 carbon atoms, such as ethylenediamine,1,3-propanediamine, 1,2-propanediamine, neopentanediamine,hexamethylenediamine, octamethylenediamine, isophoronediamine,4,4′-diaminodicyclohexylmethane,3,3′-dimethyl-4,4′-diaminodicyclohexylmethane,4,7-dioxadecane-1,10-diamine(3,3′-bis[1,2-ethanediylbis(oxy)]-1-propanamine),4,9-dioxadodecane-1,12-diamine(3,3′-bis[1,3-butanediylbis(oxy)]-1-propanamine),4,7,10-trioxatridecane-1,13-diamine(3,3′-bis[oxybis(2,1-ethanediyloxy)]-1-propanamine),2-(ethylamino)ethylamine, 3-(methylamino)propylamine,diethylenetriamine, N₃ Amine (N-(2-aminoethyl)-1,3-propylenediamine),dipropylenetriamine or N₄ Amine(N,N′-bis(3-aminopropyl)ethylenediamine); alkanolamines having up to 20carbon atoms, such as monoethanolamine, 2-amino-1-propanol,3-amino-1-propanol, 2-amino-1-butanol, isopropanolamine,2-amino-2-methyl-1-propanol, 5-amino-1-pentanol, 2-amino-1-pentanol,6-aminohexanol, methylaminoethanol, 2-(2-aminoethoxy)ethanol,N-(2-aminoethyl)ethanolamine, N-methylethanolamine, N-ethylethanolamine,N-butylethanolamine, diethanolamine, 3-(2-hydroxyethylamino)-1-propanolor diisopropanolamine; and dimercaptans or polymercaptans having up to20 carbon atoms, such as 1,2-ethanedithiol, 1,3-propanedithiol,1,4-butanedithiol, 2,3-butanedithiol, 1,5-pentanedithiol,1,6-hexanedithiol, 1,8-octanedithiol, 1,9-nonanedithiol,2,3-dimercapto-1-propanol, dithiothreitol, dithioerythritol,2-mercaptoethyl ether or 2-mercaptoethyl sulfide. Further suitable chainextenders include oligomeric compounds having two or more of theabovementioned reactive functional groups, examples beinghydroxyl-containing oligomers, such as polyethers, polyesters orhydroxyl-containing acrylate/methacrylate copolymers. Oligomeric chainextenders are extensively described in the literature and generally havemolecular weights in the range from 200 to 2000 daltons. Preferred chainextenders are the diols or polyols having up to 20 carbon atoms,especially the aliphatic diols having 6 to 20 carbon atoms, examplesbeing ethylene glycol, diethylene glycol, neopentyl glycol, and1,6-hexanediol.

Component C further includes compounds C2 which flexibilize the UV-curedcoating. Flexibilization can be achieved, inter alia, by reacting atleast some of the free isocyanate groups of the binder with hydroxyalkylesters and/or alkylamine amides of relatively long-chain dicarboxylicacids, preferably aliphatic dicarboxylic acids having at least 6 carbonatoms. Examples of suitable dicarboxylic acids are adipic acid, sebacicacid, dodecanedioc acid, and/or dimeric fatty acids. The flexibilizationreactions may in each case be carried out before or after the additionof component B onto the isocyanato-containing prepolymers.Flexibilization is also achieved by using relatively long-chainaliphatic diols and/or diamines, especially aliphatic diols and/ordiamines having at least 6 carbon atoms, as chain extenders C1.

The coating composition may further comprise one or more reactivediluents. Reactive diluents are liquid compounds of low molecular masswhich have at least one, polymerizable, ethylenically unsaturated doublebond. An overview of reactive diluents can be found, for example, in J.P. Fouassier (ed.), Radiation Curing in Polymer Science and Technology,Elsevier Science Publisher Ltd., 1993, Vol. 1, pp. 237-240. Preferenceis given to reactive diluents R based on esters of acrylic acid and/ormethacrylic acid with aliphatic diols or polyols, at least two of the OHgroups of the diols or polyols having been esterified with acrylicand/or methacrylic acid (reactive diluents R). Suitable aliphatic diolsor polyols generally have 2 to 20 carbon atoms and may have a linear,branched or cyclic carbon framework. They contain preferably nofunctional groups. Apart from one or two ether oxygens, they preferablyhave no heteroatoms. Examples of reactive diluents of this kind arebutanediol diacrylate, hexanediol diacrylate, octanediol diacrylate,decanediol diacrylate, cyclohexanediol diacrylate, trimethylolpropanetriacrylate, pentaerythritol tetraacrylate, dipentaerythritolpenta/hexaacrylate, dipropylene glycol diacrylate, and also thecorresponding esters of methacrylic acid, and the products LR 8887, PO33F, LR 8967, LR 8982 available under the BASF brand name Laromer®.

In addition, the coating composition used in accordance with theinvention may comprise further reactive diluents other than theabovementioned reactive diluents R. Such reactive diluents are likewisemono-, di- or polyunsaturated compounds. They usually serve to influencethe viscosity and the coatings properties, such as the crosslinkingdensity, for example. Examples of such compounds are (meth)acrylic acidand the C₁-C₁₀ alkyl esters thereof, maleic acid and its C₁-C₁₀ alkylesters and monoesters, vinyl acetate, vinyl ethers, divinylureas,polyethylene glycol di(meth)acrylate, vinyl (meth)acrylate, allyl(meth)acrylate, styrene, vinyltoluene, divinylbenzene,tris(acryloyloxymethyl) isocyanurate, ethoxyethoxyethyl acrylate,N-vinylpyrrolidone, phenoxyethyl acrylate, dimethylaminoethyl acrylate,hydroxyethyl (meth)acrylate, butoxyethyl acrylate, isobornyl(meth)acrylate, dimethylacrylamide and dicyclopentyl acrylate, and alsothe long-chain linear diacrylates described in EP 0 250 631 A1 having amolecular weight of from 400 to 4000, preferably from 600 to 2500daltons. Also suitable for use, moreover, is the reaction product of 2mols of acrylic acid with one mol of a dimeric fatty alcohol generallyhaving 36 carbon atoms. Mixtures of said monomers are also suitable.

The scratch-resistant, weathering-stable coating compositions generallyinclude from 0.1 to 5% by weight, preferably from 0.1 to 2% by weight,in particular from 0.2 to 1% by weight, of at least one photoinitiator,which is able to initiate the polymerization of ethylenicallyunsaturated double bonds. Such initiators include benzophenone andderivatives of benzophenone, such as 4-phenylbenzophenone and4-chlorobenzophenone, Michler's ketone, anthrone, acetophenonederivatives, such as 1-benzoylcyclohexan-1-ol,2-hydroxy-2,2-dimethylacetophenone and2,2-dimethoxy-2-phenylacetophenone, benzoin and benzoin ethers, such asmethyl, ethyl and butyl benzoin ether, benzil ketals, such as benzildimethyl ketal,2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one, anthraquinoneand its derivatives, such as β-methylanthraquinone andtert-butylanthraquinone, acylphosphine oxides, such as2,4,6-trimethylbenzoyldiphenylphosphine oxide, ethyl2,4,6-trimethylbenzoylphenylphosphinate, and bisacylphosphine oxides.Initiators of this kind are, for example, the products availablecommercially under the brand names Irgacure® 184, Darocure® 1173 fromCiba Geigy, Genocure® from Rahn, or Lucirin® TPO from BASF AG. Preferredphotoinitiators also include phenylglyoxalic acid, its esters and itssalts, which may also be used in combination with one of theabovementioned photoinitiators. For further details reference may herebybe made to German Patent Application P 198 267 12.6 in its entirety.

Furthermore, depending on their intended use, the radiation-curableformulations of the invention may contain up to 35% by weight ofcustomary auxiliaries, such as thickeners, leveling assistants,defoamers, UV stabilizers, lubricants, and fillers. Suitable auxiliariesare sufficiently well known to the skilled worker from the technology ofpaints and coatings. Appropriate fillers comprise silicates, e.g.,silicates obtainable by hydrolysis of silicon tetrachloride, such asAerosil® from Degussa, siliceous earth, talc, aluminum silicates,magnesium silicates, calcium carbonates, etc. Suitable stabilizersinclude typical UV absorbers, such as oxanilides, triazines andbenzotriazole (the latter obtainable as Tinuvin® grades fromCiba-Spezialitätenchemie), and benzophenones. These may be used alone ortogether with appropriate free-radical scavengers, examples beingsterically hindered amines, such as 2,2,6,6-tetramethylpiperidine,2,6-di-tert-butylpiperidine or derivatives thereof, e.g.,bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate. Stabilizers are usedcommonly in amounts of from 0.1 to 5.0% by weight and preferably from0.5 to 3.5% by weight, based on the coating composition.

Based on the overall weight of the coating composition, excludingpigments and fillers, the coating compositions used in accordance withthe invention preferably contain:

10-95% by weight, in particular from 20 to 90% by weight and especiallyfrom 30 to 80% by weight, of at least one aliphatic urethane(meth)acrylate prepolymer PU;

5-90% by weight, in particular from 10 to 80% by weight and especiallyfrom 20 to 70% by weight, of at least one reactive diluent R;

0.1-5% by weight of at least one photoinitiator; and, if desired,

0-20% by weight of further reactive diluents, and

0-15% by weight, preferably 2-9% by weight, of additives customary forcoating compositions.

The total amount of reactive diluent is preferably not more than 80% byweight. If a diacrylate or dimethacrylate is used as reactive diluent R,the coating composition contains in particular from 20 to 80% by weightof reactive diluent and especially this reactive diluent R as solereactive diluent. If a compound having more than two acrylate ormethacrylate groups is used as reactive diluent R, then R is usedpreferably in an amount of from 5 to 60% by weight, more preferably from10 to 50% by weight.

The above-described coating compositions are liquid formulations and assuch may be applied in the customary manner without addition ofsolvents.

The inventive coating compositions are particularly used in the form ofclearcoats, so that they normally contain no fillers, or onlytransparent fillers, and no hiding pigments. Use in the form ofpigmented coating compositions is, however, also possible. In that casethe coating compositions contain from 2 to 40% by weight, based on theoverall weight of the coating composition, of one or more pigments.

Furthermore, in this case the coating compositions may contain from 1 to30% by weight, based on the overall weight of the coating composition,of one or more fillers.

The coatings of the invention may also be applied in aqueous form. Forthis purpose, either some of the isocyanate groups are reacted withmolecules having an isocyanate reactive group and a hydrophilic,stabilizing group, such as carboxylate or sulfonate, examples beingdimethylolpropionic acid, hydroxypivalic acid, and sarcosine.Subsequently, the reaction product is neutralized with alkali or aminesand the binder is dispersed in the coating composition or emulsifiedwith protective colloids.

The coating compositions of the invention may be applied to glass andthe various metal substrates, such as, for example, aluminum, steel,various ferrous alloys, and the like. They are preferably used as aclearcoat or topcoat, for example, in the coil-coating segment and inthe segment of automotive topcoats.

The coating compositions may also be applied to other substrates, suchas wood, paper, plastics, mineral substrates, and the like. Furthermore,they may be used as a coating on packaging containers and also forcoatings for films, for example, for the furniture industry.

To produce coatings on metal substrates, the coating compositions of theinvention are applied preferably to primed or basecoated metal panels ormetal strips. The commonly used basecoat materials may be used asprimers. Both conventional and aqueous basecoat materials are employed.Further, it is also possible to apply the coating compositions of theinvention to metal substrates which are first coated with anelectrodeposition coating and subsequently coated with a functional coatand wet-on-wet with a basecoat material. In the case of said processesit is generally necessary for the basecoat material and the surfacerand/or the functional coat to be baked before the coating composition ofthe invention is applied.

The process of the invention for producing multicoat coating systemscomprises the following steps:

applying a preferably pigmented basecoat material to a substratesurface;

drying and/or crosslinking the basecoat film;

applying a topcoat material; and

curing the topcoat by exposure to UV light under an inert gasatmosphere.

The topcoat material used here is a coating composition based onurethane (meth)acrylate prepolymers PU, as described above.

The coating films are cured by means of UV radiation under an inert gasatmosphere. The installations and conditions for such curing methods areknown from the literature (cf., e.g., Holmes, UV and EB CuringFormulations for Printing Inks, Coatings and Paints, SITA Technology,Academic Press, London, United Kingdom 1984).

The coatings obtainable in accordance with the invention are notable forgood surface hardness and a higher scratch resistance thanconventionally cured coatings. They are also notable for improvedchemical resistance.

The chemical resistance is normally tested with the chemicals with whichthe coating may come into contact. In the case of wood coatings, theseare, for example, typical household chemicals, such as coffee, mustardor red wine, whereas coatings for motor vehicles are tested for theirresistance to gasoline, acids, alkalis, tree resin, etc. The coatings ofthe invention exhibit a markedly improved resistance to theaforementioned chemicals.

In the text below, the invention is illustrated with reference toworking examples. All parts in the examples are by weight unlessexpressly stated otherwise.

1. Preparation of the Urethane Acrylate Coating Compositions

The coating compositions are prepared from the components indicated inTable 1 by thorough stirring using a dissolver or a stirrer. Films wereproduced on cleaned glass plates using a box-type coating bar, gap size200 μm. The films are cured in an IST coating unit (type M 402×1-R-IR-SLC-So inert) with 2 UV emitters (high-pressure mercury vaporlamps type M 400 U2H and type M400 U2HC) and a conveyor belt runningspeed of 10 m/min. The radiation dose is approximately 1800 mJ/cm².

TABLE 1 Composition of coating compositions 1-11 Coating LR 8987 THEICIrgacure composition (1) (2) PHA (3) HDDA (4) 184 (5) 1 100 4 2 90 10 43 70 30 4 4 50 50 4 5 95 5 4 6 90 10 4 7 80 20 4 8 50 50 4 9 80 20 4 1050 50 4 11 20 80 4 (1) Laromer ® LR 8987: commercial mixture of analiphatic urethane acrylate containing 30% by weight hexanedioldiacrylate, from BASF AG. Molecular weight approximately 650 g/mol,Functionality approximately 2.8 double bonds/mol (about 4.5 mol/kg),Viscosity 2-6 Pa · s (DIN EN ISO 3219). (2) THEIC: triacrylate oftri(hydroxyethyl)cyanurate (3) PHA: dipentaerythritol penta/hexaacrylate(4) HDDA: hexanediol diacrylate (5) Irgacure  ® 184 from Ciba Geigy,commercial photoinitiator.

2. Determination of the Mechanical Stability

The pendulum hardness in accordance with König, DIN 53 157, ISO 1522,the Erichsen indentation in accordance with DIN 53 156, ISO 1520 andscratch resistance using the Scotch Brite test after storage for 24hours in a controlled-climate chamber were determined for the coatingcompositions listed in Table 1.

To determine the scratch resistance, a film was applied to a cleanedglass plate colored black. This permits the loss of gloss to bedetermined following corresponding stress. Curing with UV radiation wascarried out under a nitrogen atmosphere (Table 2a) and also under air(Table 2b).

In the Scotch Brite test, the test specimen is a 3×3 cm siliconcarbide-modified fiber nonwoven (Scotch Brite SUFN, 3M Deutschland,41453 Neuβ) mounted on a cylinder. This cylinder presses the fibernonwoven against the coating under a load of 750 g and is moved over thecoating pneumatically. The path of the deflection is 7 cm. After 10 or50 double strokes (DS), the gloss (6-fold determination) in the centralregion of the stress is measured in analogy to DIN 67530, ISO 2813 at anincident angle of 60° and the difference from the gloss value prior totreatment is formed (Δ gloss value). The Δ gloss value is inverselyproportional to the scratch resistance.

TABLE 2a Test results of coatings 1 to 11 on curing under a nitrogenatmosphere (O₂ ≦ 500 ppm, determined by means of a Galvanoflux probe -electrochemical cell based on a lead/lead oxide redox couple) Scratchresistance¹⁾ Pendulum Erichsen (Δ gloss value) attenuation²⁾indentation³⁾ Example 10 DS 50 DS (s) (mm) 1 4.5 6.1 172 3.5 2 3.3 6.3176 0.7 3 5.4 9.1 189 0.9 4 3.9 6.0 189 0.9 5 3.1 5.1 181 0.9 6 2.7 4.3175 0.8 7 2.2 3.7 183 0.6 8 0.6 1.3 182 0.5 9 3.7 5.5 178 2.3 10 2.7 6.0172 1.2 11 2.0 5.1 185 0.8

TABLE 2b Test results of coatings 1 to 11 on curing under air (ExamplesV1 to V11) Scratch resistance¹⁾ Pendulum Erichsen (Δ gloss value)attenuation²⁾ indentation³⁾ Example 10 DS 50 DS (s) (mm) V1 38.2 33.8175 3.3 V2 31.4 39.1 171 3.0 V3 21.9 39.4 186 3.2 V4 20.5 35.6 186 1.5V5 26.2 37.5 176 2.5 V6 30.0 40.8 171 2.1 V7 22.5 35.0 182 0.8 V8 8.314.8 175 0.6 V9 40.6 41.5 165 3.0  V10 n.m.⁴⁾ n.m.⁴⁾ 174 2.2  V11 n.m.⁴⁾n.m.⁴⁾ n.m.⁴⁾ n.m.⁴⁾ ¹⁾Δ gloss after Scotch-Brite treatment after 10 and50 double strokes (DS), respectively ²⁾König pendulum hardness, DIN 53157, ISO 1522 ³⁾Erichsen indentation, DIN 53 156, ISO 1520 ⁴⁾notmeasurable

3. Testing of Resistance to Chemicals

The coatings with the compositions of Examples 1 and 8 from Table 1 areapplied in a film thickness of approximately 40 μm to a metal panel andcured under the stated atmosphere with a belt speed of 10 m/min under120 W lamps. The stated chemicals were applied dropwise in series andthe panel was placed in a gradient oven with a temperature zone of 25 to90° C. for one hour. The table reports the lowest temperature at whichdamage occurs to the film surface; inspection after 24 hours.

TABLE 3 Chemical resistance on curing under air and under nitrogenExample (atmosphere) 1 (Air) 1 (Nitrogen) 8 (Air) 8 (Nitrogen) Water 7575 75 75 Tree resin 34 75 75 75 Pancreatin/Water 1:1 51 63 36 60Sulfuric acid 1% 42 66 67 66

135/ew/sg

We claim:
 1. A process for producing a scratch-resistant coating, saidprocess comprising: applying at least one UV-curable coating compositioncomprising a mixture of at least one aliphatic urethane (meth)acrylateprepolymer having at least two double bonds per molecule and having aviscosity in the range from 250 to 11,000 mPa.s, and at least onereactive diluent, to a substrate to form a wet coating and curing saidwet coating by exposure to ultraviolet radiation under an inert gasatmosphere, wherein the aliphatic urethane prepolymer is obtained byreacting at least 25% of the isocyanate groups of a compound (A)containing at least two isocyanate groups per molecule with (B) at leastone hydroxy alkyl ester of acrylic acid, methacrylic acid or bothacrylic acid and methacrylic acid, and subsequently reacting anyremaining isocyanate groups of (A) with a chain extender (C) selectedfrom the group consisting of an aliphatic diol having up to 20 carbonatoms, a polyol having up to 20 carbon atoms, a diamine having up to 20carbon atoms, a polyamine having up to 20 carbon atoms, an alkanolaminehaving up to 20 carbon atoms, a dimercaptan having up to 20 carbonatoms, a polymercaptan having up to 20 carbon atoms, a hydroxyalkylesterof a long-chain dicarboxylic acid, and an alkylamineamide of a longchain dicarboxylic acid, wherein component (A) is obtained by reactingat least one of a low molecular mass aliphatic diisocyanate orpolyisocyanate with a compound having at least two isocyanate-reactivefunctional groups wherein the ratio of the isocyanate groups ofcomponent (A) to the functional groups of the compound is in the rangeof from 3:1 to 1:2.
 2. The process as claimed in claim 1, wherein saidUV-curable coating composition further comprises at least one reactivediluent selected from the group consisting of difunctional esters ofacrylic acid, difunctional esters of methacrylic acid, polyfunctionalesters of methacrylic acid, polyfunctional esters of acrylic acid withacrylic acid, diols, polyols and mixtures thereof.
 3. The process asclaimed in claim 1, wherein, based on an overall weight of the coatingcomposition, excluding pigments and fillers, the coating compositioncomprises: 5-90% by weight of at least one aliphatic urethane(meth)acrylate prepolymer; 10-95% by weight of the reactive diluent; and0.1-5% by weight of at least one photoinitiator.
 4. The process asclaimed in claim 1, wherein the urethane (meth)acrylate prepolymer has anumber-average molecular weight in the range from 500 to
 5000. 5. Theprocess as claimed in claim 1, wherein the urethane (meth)acrylateprepolymer has a double bond equivalent weight in the range from 250 to2000.
 6. The process as claimed in claim 1 wherein at least a portion ofthe free isocyanate groups of the urethane (meth)acrylate prepolymerhave been reacted with one or more molecules which contain anisocyanate-reactive group and a hydrophilic, stabilizing group.
 7. Theprocess as claimed in claim 1, wherein the coating composition furthercomprises from 2 to 40% by weight of one or more pigments, based on theoverall weight of the coating composition.
 8. The process as claimed inclaim 1, wherein the coating composition further comprises from 1 to 30%by weight of one or more fillers, based on the overall weight of thecoating composition.
 9. The process as claimed in claim 1, wherein thescratch-resistant coating is obtained by a multicoat coating process,said multicoat coating process comprising: i. applying a basecoatmaterial to a substrate surface; ii. drying and/or crosslinking thebasecoat film; iii. applying the UV-curable coating composition; and iv.curing the UV-curable coating composition by exposure to UV light underan inert gas atmosphere.
 10. The process as claimed in claim 1, whereinthe substrate has a metallic surface.
 11. The process as claimed inclaim 3, wherein the coating composition further comprises from 2 to 9%by weight of one or more additives.
 12. The process as claimed in claim5, wherein the aliphatic urethane (meth)acrylate prepolymer has a doublebond equivalent weight in the range from 300 to 900 daltons.
 13. Theprocess as claimed in claim 9, wherein the basecoat material ispigmented.
 14. The process as claimed in claim 1, wherein saidUV-curable coating composition comprises a reactive diluent comprisingat least one esterified polyol having five or more acrylate groups. 15.The process as claimed in claim 1, wherein the urethane prepolymerconsists of reacted units of (A), (B), and (C).
 16. The process asclaimed in claim 3, wherein the coating composition further comprises upto 15% by weight of one or more additives.
 17. The process as claimed inclaim 3, wherein the coating composition further comprises up to 20% byweight of one or more further diluents.
 18. A scratch resistant coatingprepared by the process as claimed in claim
 1. 19. The process asclaimed in claim 1, wherein said UV-curable coating compositioncomprises dipentaerythritol penta/hexaacrylate.
 20. A process forproducing a scratch-resistant coating, said process comprising: applyingat least one UV-curable coating composition comprising a mixture of atleast one aliphatic urethane (meth)acrylate prepolymer having at leasttwo double bonds per molecule and having a viscosity in the range from250 to 11,000 mPa.s and wherein at least a portion of the freeisocyanate groups of the urethane (meth)acrylate prepolymer have beenreacted with one or more of a hydroxyalkyl ester of an aliphaticdicarboxylic acid having at least 6 carbon atoms or an alkylamine amideof an aliphatic dicarboxylic acid having at least 6 carbon atoms, and atleast one reactive diluent, to a substrate to form a wet coating andcuring said wet coating by exposure to ultraviolet radiation under aninert gas atmosphere, wherein the aliphatic urethane prepolymer isobtained by reacting at least 25% of the isocyanate groups of a compound(A) containing isocyanate groups with (B) at least one hydroxy alkylester of acrylic acid, methacrylic acid or both acrylic acid andmethacrylic acid, and subsequently reacting any remaining isocyanategroups of (A) with a chain extender (C) selected from the groupconsisting of an aliphatic diol having up to 20 carbon atoms, a polyolhaving up to 20 carbon atoms, a diamine having up to 20 carbon atoms, apolyamine having up to 20 carbon atoms, an alkanolamine having up to 20carbon atoms, a dimercaptan having up to 20 carbon atoms, apolymercaptan having up to 20 carbon atoms, a hydroxyalkylester of along-chain dicarboxylic acid, and an alkylamineamide of a long chaindicarboxylic acid.
 21. The process as claimed in claim 20, wherein saidUV-curable coating composition further comprises at least one reactivediluent selected from the group consisting of difunctional esters ofacrylic acid, difunctional esters of methacrylic acid, polyfunctionalesters of methacrylic acid, polyfunctional esters of acrylic acid withacrylic acid, diols, polyols and mixtures thereof.
 22. The process asclaimed in claim 20, wherein, based on an overall weight of the coatingcomposition, excluding pigments and fillers, the coating compositioncomprises: 5-90% by weight of at least one aliphatic urethane(meth)acrylate prepolymer; 10-95% by weight of the reactive diluent; and0.1-5% by weight of at least one photoinitiator.
 23. The process asclaimed in claim 20, wherein the urethane (meth)acrylate prepolymer hasa number-average molecular weight in the range from 500 to
 5000. 24. Theprocess as claimed in claim 20, wherein the urethane (meth)acrylateprepolymer has a double bond equivalent weight in the range from 250 to2000.
 25. The process as claimed in claim 20, wherein the one or moreisocyanate groups of component A have been reacted in a stoichiometricratio with one or more hydroxyl groups of component B.
 26. The processas claimed in claim 20, wherein at least a portion of the freeisocyanate groups of the urethane (meth)acrylate prepolymer have beenreacted with one or more molecules which contain an isocyanate-reactivegroup and a hydrophilic, stabilizing group.
 27. The process as claimedin claim 20, wherein the coating composition further comprises from 2 to40% by weight of one or more pigments, based on the overall weight ofthe coating composition.
 28. The process as claimed in claim 20, whereinthe coating composition further comprises from 1 to 30% by weight of oneor more fillers, based on the overall weight of the coating composition.29. The process as claimed in claim 20, wherein the scratch-resistantcoating is obtained by a multicoat coating process, said multicoatcoating process comprising: i. applying a basecoat material to asubstrate surface; ii. drying and/or crosslinking the basecoat film;iii. applying the UV-curable coating composition; and iv. curing theUV-curable coating composition by exposure to UV light under an inertgas atmosphere.
 30. The process as claimed in claim 20, wherein thesubstrate has a metallic surface.
 31. The process as claimed in claim22, wherein the coating composition further comprises from 2 to 9% byweight of one or more additives.
 32. The process as claimed in claim 24,wherein the aliphatic urethane (meth)acrylate prepolymer has a doublebond equivalent weight in the range from 300 to 900 daltons.
 33. Theprocess as claimed in claim 30, wherein the basecoat material ispigmented.
 34. The process as claimed in claim 20, wherein saidUV-curable coating composition comprises a reactive diluent comprisingat least one esterified polyol having five or more acrylate groups. 35.The process as claimed in claim 20, wherein the aliphatic urethaneprepolymer comprises reacted groups of a diamine or a polyamine.
 36. Theprocess as claimed in claim 20, wherein the urethane prepolymer consistsof reacted units of (A), (B), and (C).
 37. The process as claimed inclaim 22, wherein the coating composition further comprises up to 15% byweight of one or more additives.
 38. The process as claimed in claim 22,wherein the coating composition further comprises up to 20% by weight ofone or more further diluents.
 39. A scratch resistant coating preparedby the process as claimed in claim
 20. 40. The process as claimed inclaim 20, wherein said UV-curable coating composition comprisesdipentaerythritol penta/hexaacrylate.
 41. A process for producing ascratch-resistant coating, said process comprising: applying at leastone UV-curable coating composition comprising a mixture of at least onealiphatic urethane (meth)acrylate prepolymer comprising reacted groupsof a diamine or a polyamine and having at least two double bonds permolecule and having a viscosity in the range from 250 to 11,000 mPa.s,and at least one reactive diluent, to a substrate to form a wet coatingand curing said wet coating by exposure to ultraviolet radiation underan inert gas atmosphere, wherein the aliphatic urethane prepolymer isobtained by reacting at least 25% of the isocyanate groups of a compound(A) containing isocyanate groups with (B) at least one hydroxy alkylester of acrylic acid, methacrylic acid or both acrylic acid andmethacrylic acid, and subsequently reacting any remaining isocyanategroups of (A) with a chain extender (C) selected from the groupconsisting of an aliphatic diol having up to 20 carbon atoms, a polyolhaving up to 20 carbon atoms, a diamine having up to 20 carbon atoms, apolyamine having up to 20 carbon atoms, an alkanolamine having up to 20carbon atoms, a dimercaptan having up to 20 carbon atoms, apolymercaptan having up to 20 carbon atoms, a hydroxyalkylester of along-chain dicarboxylic acid, and an alkylamineamide of a long chaindicarboxylic acid.
 42. The process as claimed in claim 41, wherein saidUV-curable coating composition further comprises at least one reactivediluent selected from the group consisting of difunctional esters ofacrylic acid, difunctional esters of methacrylic acid, polyfunctionalesters of methacrylic acid, polyfunctional esters of acrylic acid withacrylic acid, diols, polyols and mixtures thereof.
 43. The process asclaimed in claim 41, wherein, based on an overall weight of the coatingcomposition, excluding pigments and fillers, the coating compositioncomprises: 5-90% by weight of at least one aliphatic urethane(meth)acrylate prepolymer, 10-95% by weight of the reactive diluent; and0.1-5% by weight of at least one photoinitiator.
 44. The process asclaimed in claim 41, wherein the urethane (meth)acrylate prepolymer hasa number-average molecular weight in the range from 500 to
 5000. 45. Theprocess as claimed in claim 41, wherein the urethane (meth)acrylateprepolymer has a double bond equivalent weight in the range from 250 to2000.
 46. The process as claimed in claim 41, wherein the one or moreisocyanate groups of component A have been reacted in a stoichiometricratio with one or more hydroxyl groups of component B.
 47. The processas claimed in claim 41, wherein at least a portion of the freeisocyanate groups of the urethane (meth)acrylate prepolymer have beenreacted with one or more molecules which contain an isocyanate-reactivegroup and a hydrophilic, stabilizing group.
 48. The process as claimedin claim 41, wherein the coating composition further comprises from 2 to40% by weight of one or more pigments, based on the overall weight ofthe coating composition.
 49. The process as claimed in claim 41, whereinthe coating composition further comprises from 1 to 30% by weight of oneor more fillers, based on the overall weight of the coating composition.50. The process as claimed in claim 41, wherein the scratch-resistantcoating is obtained by a multicoat coating process, said multicoatcoating process comprising: i. applying a basecoat material to asubstrate surface; ii. drying and/or crosslinking the basecoat film;iii. applying the UV-curable coating composition; and iv. curing theUV-curable coating composition by exposure to UV light under an inertgas atmosphere.
 51. The process as claimed in claim 41, wherein thesubstrate has a metallic surface.
 52. The process as claimed in claim43, wherein the coating composition further comprises from 2 to 9% byweight of one or more additives.
 53. The process as claimed in claim 45,wherein the aliphatic urethane (meth)acrylate prepolymer has a doublebond equivalent weight in the range from 300 to 900 daltons.
 54. Theprocess as claimed in claim 50, wherein the basecoat material ispigmented.
 55. The process as claimed in claim 41, wherein saidUV-curable coating composition comprises a reactive diluent comprisingat least one esterified polyol having five or more acrylate groups. 56.The process as claimed in claim 41, wherein the urethane prepolymerconsists of reacted units of (A), (B), and (C).
 57. The process asclaimed in claim 43, wherein the coating composition further comprisesup to 15% by weight of one or more additives.
 58. The process as claimedin claim 43, wherein the coating composition further comprises up to 20%by weight of one or more further diluents.
 59. A scratch resistantcoating prepared by the process as claimed in claim
 41. 60. The processas claimed in claim 41, wherein said UV-curable coating compositioncomprises dipentaerythritol penta/hexaacrylate.